Not really encounters:
- Years ago (2006-ish) I emailed Martin Freeman and his brother gave a nice reply on his behalf.
- Saw Michael J. Fox at an event and was within five metres of him when he passed by. Can confirm he's short.
- Worked over a year online with Jeffrey Weissman (replaced Crispin Glover as George McFly in the Back to the Future sequels) while developing a script written by a mutual friend. He made me and my cowriter do five drafts over 1.5 years before scrapping the project. I'm still bitter about this. He was kind of a dick.
- Stalked Keanu's house when he wasn't home and left him a note written on a napkin, which was all the paper I had around. Keanu also answered one of my questions on his Reddit AMA where I asked why he had so many characters named John and he said he'd just been wondering the same thing.
- Saw Zachary Quinto twice at events - once from a very far distance at the Star Trek Into Darkness premiere, and once at a late movie screening from his production company with FAQ, where he also passed by within a few metres of me. He looked like he really wanted to go to bed but was trying hard to remain professional and not be too cranky, and the technical failures were testing his patience. Later I managed to send him a letter via a mutual friend who was collecting letters and stuff from fans to create a scrapbook to pass to him. He told her the scrapbook was really great and he loved it.