Cohen used both characters to conduct outrageous interviews with unsuspecting people who thought Ali G and Borat were on the level.
Now that his alter-egos are so popular, he can't expect the same comedic results.
"When I was being Ali G and Borat I was in character sometimes 14 hours a day and I came to love them, so admitting I am never going to play them again is quite a sad thing," Cohen told The Telegraph in a rare interview, speaking as himself and not one of his characters.
"It is like saying goodbye to a loved one," he added. "It is hard, and the problem with success, although it's fantastic, is that every new person who sees the 'Borat' movie is one less person I 'get' with Borat again, so it's a kind of self-defeating form, really. It's upsetting, but the success has been great and better than anything I could have dreamed of."
Cohen can now be seen in "Sweeney Todd."