That sounds simple enough, doesn't it?
What Luck most wanted was to satisfy himself as to whether Rosemary could possibly play the part of old Dave's daughter.

If she could, he would sleep sounder that night; if she could not,--Luck was not at all clear as to what he should do if she failed.

He told her just where to walk into the "scene," which is the range of the camera.

He went down part way to the corral and drew a line with his toe, and told her to stop when she reached that line and to look away up the trail which wound down among the rocks and sage.

When he called to her she was to turn and walk back, trying to imagine that she was much worried and disappointed.
"Your dad was to have come last night," Luck suggested.