It must be so easy on the radio. You don't have to learn the lines. How many times have I heard that. The truth is different. For a start, most films have only some blue-screen effects.
Radio is all blue-screen.
The actors have to imagine everything. Pretending you are on the lushly verdant Moon of Endor when you are, in fact, in a tiny mirrored booth on Beverley Boulevard takes some doing. As for learning lines, if you aren't absolutely familiar with them and their position on the script page, you'd never be able to take your eyes off it. If you don't know the script, you're bound to lose your place, with a resulting, embarrassing pause. My problem is that I never learned to write neatly.