Screenplays and novel writing

I'm currently turning my novel Replica into a screenplay. Why, I hear you cry - after all, if it's difficult to sell a novel which will cost a publisher £10,000 to produce (Snowbooks' figures), how much harder is it to sell a screenplay which will cost up to £500 an hour to make? (That's BBC tarifs - I couldn't find the total cost of a made-for-TV 90 minute film.)

Partly it's because I think Replica would transfer well to the small screen, and partly because it's an interesting challenge. Screenplay writing is very different from novel writing; since there will be input from the director, actors, set and costume designers, on the page it's the dialogue that does all the work. You don't say what the characters are feeling; you don't write backstory; you don't tell the actors how to deliver the lines. So the dialogue has to be very good indeed.

You lay the screenplay out to an accepted format, in Courier 12, as in the picture. This enables the length of a script to be assessed quickly. A film should come to no more than 120 pages. I'm a concise writer, so it's a new sensation for me to have to cut scenes and dialogue. I now realize why adaptations often have whole subplots missing, and why films can be so different from the original novel.

I've a nasty feeling it probably takes as long to become proficient at writing screenplays as it does to write readable novels.