Justin Theroux is one year into his union with Jennifer Aniston, and he's happily married. His secret to success: kindness.The Girl on the Train star recently spoke to People about the key to a lasting relationship. At first, he started to rattle off about communication, then he settled on the tip that's worked best for him."Being kind, that's it," he told People. "I think that's the best one: Approach each situation with kindness."In the film, out October 7 , Theroux plays a husband who struggled with this piece of advice. As Tom Watson, he's plagued by the frantic actions of his ex-wife Rachel, played by Emily Blunt. However, not everything with Tom is as it seems: their marriage ended because of Rachel's alcoholism and Tom's affair.Though, there's another high-profile marriage that Theroux (and Aniston) have gotten tangled up in: Brangelina's breakup. The New York Post ran a photo of Aniston on its cover reporting the split, and Friends co-star Courteney Cox cleared the air and said that whatever's happening with Brangelina doesn't involve Aniston. Earlier this week, Theroux even spoke out , saying that he's a child of divorce and the news of the breakup is terrible for the couple's six children.See the full clip of Justin Theroux's expert advice below: