How to Stalk Someone Stealthily Outdoors
If you want to stealthily stalk a friend for fun, like to gather information for their surprise present or party, you'll need to blend in and be discreet. Blend into the crowd by wearing clothing that will help you look like everyone else. For example, if you're stalking your friend at the beach, wear a bathing suit. Another way to blend in is to act like you belong. If your friend is shopping, you should also appear to be shopping. Instead of staring at your friend, which may look suspicious, use your peripheral vision or glance at them from time to time. If your friend catches you, don't try to run away. Instead, have an explanation ready, like you're walking home from your aunt's house.
Wear appropriate clothing. When stalking your friend, be sure to dress for the weather. You don't want to stand out with a trench coat and an elaborate mustache disguise. Use common sense: if you're stalking your friend at a beach in the hot summer, wear a bathing suit; if you're stalking him/her in the rain, wear a rain jacket and carry an umbrella. If you're stalking at night, wearing dark clothing may help you remain unseen; however, in the event that you are spotted, you will also seem more suspicious. Many people walk/jog/run at night, so athletic clothing may be a good choice. Don't dress flashy: more on the side of muted colors. Don't overthink your attire: dress like everyone else. Consider a slight disguise. Your friend obviously knows what you look like and how you normally dress, so you'll probably want to alter your appearance slightly. Depending on how determined you are, you could go to a thrift store and buy some inexpensive clothing so that your friend won't recognize your usual style. Wearing a hoodie can be effective, but it can also make you stand out, particularly when wearing a hoodie up draws attention. If you normally wear your hair a certain way, try styling it differently. If it doesn't feel like you, that's a good thing because it probably doesn't look like you either. If you're not usually a hat person, wearing a hat will definitely make you unrecognizable to your friend at a quick glance. Don't go for an elaborate disguise: less is more in this case. You don't want to draw attention to yourself. Obscure your face. Your friend is going to recognize your face, so consider wearing something to slightly obscure it. If it's sunny, consider buying a cheap pair of sunglasses to cover your eyes. If it's cold out, wear a scarf that can cover the bottom half of your face. If you wear make-up, perhaps apply it differently, maybe thicker. Bring some supplies. In some cases, you may need additional supplies for your stealth operation. Here are some supplies you might consider bringing along in a backpack: Binoculars. Although good for observing from long distances in nature, in urban areas binoculars will probably stand out. A long-distance lens. A camera with a long-distance lens is good for observing your friend from afar just as long as you're not always focused on him/her: make it look like you're an amateur photographer taking pictures. A spare outfit. If your friend spots you but doesn't recognize you, that's a good time to change your outfit (and personal effects) because if he/she spots you again in the same clothing he/she will definitely become suspicious. Prepare an explanation. Before you start stalking your friend, think up a reason for why you are where you are. In the event your friend notices and confronts you, this will be your explanation. This is especially important when stalking at night, when there are fewer people around to blend in with and there are fewer reasons to be outside. Here are some ideas for explanations: you dropped something; you're looking for lost pet; you're lost; you're walking home (from somewhere). Act appropriately. Where you're stalking your friend (e.g. a beach, a store, a restaurant) should determine your behavior -- which should appear natural to the environment. If your friend is shopping, you, too, should appear to be shopping. But if your friend is shopping at store where you might stand out depending on the "gender base" (e.g. for women: a Victoria's Secret; for men: a men shoe store), then perhaps a better idea would be to wait casually outside for him/her to emerge. If you don't know how to act in a certain place, remember: do what everyone else is doing. Use cover casually. This doesn't mean crouching behind bushes or diving into garbage cans. Rather, use cover appropriately for the situation. Here are some examples: If you're at a bookstore and your friend is approaching your direction, lower your face into the book as though you were short-sighted. If you're at the park, leaning behind a tree is normal behavior -- so use that tree for cover! If your environment offers no suitable cover, use your phone: hold it low and bow your head to hide your face. People do this all the time, so you'll look normal. Remember that you never know who else might be watching you , so avoid suspicious crouching and hiding even when you think you're unseen. Appear engaged. Even if you're only following your friend walking down the street, appear engaged in either your thoughts or the sights. Very few people stare straight ahead while they walk, so don't always keep your eyes trained on your friend. As it is seldom done and will likely broadcast your stalking hijinks, eye contact is the wrong kind of engagement, especially with the friend whom you are stalking. Keep your distance. Approaching too closely increases the risk of your friend spotting you, so keep just close enough so that you can always see them. If you're following your friend by car, keep a few car distances behind you and them, with cars in between. Remember, all you need is enough of a distance to see where they're headed and not to lose them if they turn. If you're in nature (e.g. the woods), one way of keeping your distance while observing your friend is to use binoculars or a long-distance camera lens. A greater distance between you and your target might be necessary at night due to the absence of other people. Keep a "soft focus" on your friend. Rather than a "hawk's gaze," you should keep a "soft focus" on your friend because you don't want to alert suspicion or make eye-contact if they were to turn toward you unexpectedly. A "soft focus" utilizes your peripheral vision, so that where your nose is pointing isn't necessarily where you're actually looking. Be quiet and stealthy. Managing the sound you make is most important when you're stalking at night, as there typically are fewer people around at that time. Walking quietly is largely a matter of balance: keep your legs bent and practice slow, heel-to-toe steps on the outer edge of your feet. Watch where you step so that you don't accidentally step on something, such as a branch, and alert your friend to your presence. Don't run away. If you see that your friend has spotted and identified you, running away will only raise his/her suspicions when you don't really know if they're even suspicious yet. Show surprise. Don't overdo it, but be surprised to have run into them. If you're feeling really confident, you could even act like you're delighted to see them, which will probably shake them of the suspicion that you were following them. Use your planned explanation. When your friend asks why you are where you are, give your explanation that you planned beforehand. Act incredulous. If your friend doesn't believe your planned explanation, your last chance at saving face is to act incredulous at their incredulity. If you get the urge to laugh, let it out because the situation is indeed ridiculous. Tell the truth. If your friend refuses to believe your planned explanation and your incredulity, the jig is probably up and it's time to tell them the truth about what you were doing. 
Wear appropriate clothing. Consider a slight disguise. Obscure your face. Bring some supplies. Prepare an explanation. Act appropriately. Use cover casually. Appear engaged. Keep your distance. Keep a "soft focus" on your friend. Be quiet and stealthy. Don't run away. Show surprise. Use your planned explanation. Act incredulous. Tell the truth. 