Writing a recommendation letter for a senior colleague A good colleague of mine from a previous life is applying for outside professorships at both pre-and-post tenure Associate levels. I was flattered when they reached out asking for a letter from me, and am happy to help.  The only thing is, they are a few more years more senior than me; I am a new, young, relatively no-name junior professor, and I worked under this person a few years ago when I was a postdoc. We had a good working relationship, the only thing is that I'm not totally familiar with the idea of writing letter for a senior colleague. My understanding of this kind of letter in general is that you get top names in your field to vouch for you. Assuming that my colleague is already recruiting top names alongside mine, what do I have to contribute that they don't; how should I approach the letter? Am I just writing a character reference, or do I wax about their potential to transform the field? Is it on me to be explicit about what I do and don't feel credible to claim about this person, or my vision for how the information in my recommendation should complement that from the other letters; or can I trust any readers to gather that info for themselves?

Because norms differ so much by discipline, please include your own field in your answer. I'm in information science.
The fact you are more junior does not mean you can't write a helpful letter.

Write a letter that describes what you observed, e.g., in his lectures and in his interactions with students and colleagues, and in his research.  Try to be specific.  Instead of simply saying he was a great lecturer, describe the techniques you observed, e.g., that he mixed PowerPoint and whiteboard effectively, answered students' questions really well, how he's mentored and taught you, etc.

After you've outlined the facts that might support it, offer your opinion, that this is an individual who would be perfect for the position.

I'm a lecturer in CS at University of Michigan.  I've written lots of LORs for both students and colleagues.  They're very formulaic.  You can do this.