Gary Burghoff was "Radar" in the movie as well, and he was still Col. Blake's clerk.  The character didn't become the ultra-naive, nehi-drinking farmboy until after the TV series had been on the air for awhile.  In the first season, he was more like the movie's Radar, and did things like smoke & drink & try to pick up women, which he seemed to have trouble doing in later episodes. (But then again, in the early shows, Hawkeye had a sister & mother and a different home state.  Later, it was firmly established that he was the only child of a widower and hailed from Maine)