I've had success with Robert Knepper. I don't collect autographs or anything so this was the second time in my life I sent someone fanmail (first time, Edward Norton back in '99, got a letter in return and he'd even written the address on the envelope himself) and I was happy to get a reply even though RK didn't write a letter like Edward Norton.
Details: I don't remember which address I sent it to, but this was the return address:
Knepper
c/o Innovative
1505 Tenth St.
Santa Monica CA
90401
I had only included some stamps with my letter (since I don't collect autographs I didn't know exactly what to do) and I got a glossy photo of T-Bag in return signed with "Thank you, amai. xoxo Robert Knepper"
I sent my letter in June and the envelope the photo was sent in was post stamped in Dallas, Texas in the middle of October.