Let's say you are mentoring a spy thriller. 

Your author has the general plot outline and you think it's a good start. They also have the MC and a few of the characters worked out. In the early stages, you need to talk about all of these things. And ask questions.

The Beginning

Plot: a terrorist group is going to pirate a ship with plutonium on it so they can sell it to a rogue state for millions. 

Example questions: Why plutonium? What are these terrorist really interested in, when it comes down to it? Are they idealists, or would they sell their grannies for a pint of lager? Where is this ship going and why is it carrying plutonium? Who is being robbed? Why? Is there a personal beef involved or is it all business? How are they going to transport this stuff? What's your planned body count? Is the rogue state part of the plot or is that inconsequential? 

The other angle is the author's reasons for writing this story: Why a spy novel? Why have you chosen this particular group? What do you want to say with your story? Are you making a political statement or are you interested in the adventure? Are you going to anger any group by assigning them the 'bad guy' role randomly*? What does your story imply on an ethical, moral and humane level*?

This will tell you the goal for your writer. What is at stake for them. Characters are not the only ones with something at stake! 

(*Note: often writers forget, or don't care about, the implications of their stories. They want to entertain or get rich and famous. If people are insulted, or if they plant destructive, negative ideas in people's minds, they don't care. Another job of a mentor is to get their author to at least consider the ethical and social impact of their story and sensitize them to their authorial responsibility.)

Once you've hashed through the plot and goal, go for the characters.

Example questions: Why is Brad divorced? How do you plan to use that in the story for maximum effect? How does the fact he's divorced affect his behavior in the story? What about Agent Richards' ailing mother? What significant limitations will that realistically set on her action range in the story? Would another handicap be more useful? What are some possibilities for Agent Kowalsky's army past, beyond the obvious?  How would Brad get himself on the team, if he's known to have personal problems?

Your author may not have the answers to any of these questions, or only very basic ones. Good. That's what they need you, their mentor for. Help them see and discover as many facets and possibilities in their story as they can. And then help them make wise, concrete decisions on which ones to follow. 

The Middle

In the middle of writing, for each section your author is planning, do this same thing. Ask, ask and ask again. Get them thinking. Get the gears in their mind whirling. Open up their horizons and turn them on to possibilities. Check and make sure their decisions are taking them towards the goal they wanted and make logical sense. 

Remember when I said "no author homework"? Writing assignments aren't homework. And in the early middle of a story, writing assignments can be quite beneficial. That is, have your author write some 'shorts' or 'vignettes' about different aspects of the plot, setting, characters, symbolism or anything else you think they would progress by doing. These little assignments can then be used for inspiration, or parts lifted wholesale and inserted into the main narrative. They are never a pointless Mickey Mouse exercise. 

At the mid-point, ask your author to stop and look at the entire story again. This may be as a mini-break in writing or as a planned mentor-author session.  Ask your author how they FEEL about the story so far. Is it going in a direction they approve of? Can they see anything they feel isn't quite right. Now is a time to switch to feeling mode for a bit. Enthusiasm for the last section needs to be gained and that happens best when the author can see how far they've come...and how good it is!! 

The End

The ending of a story is the second most important point after the beginning. Keep asking your author questions. Why does the frigate with Captain Blake blow up, and could he be got rid of in a better way? How are they going to get Agent Richards all the way up on the highest deck for the final fist-fight? Why the shark circling in the water below...isn't that a touch...melodramatic? Why not mention Jaws earlier? 

Remind your author that they are nearing their destination. All the threads are coming together, so they will need to concentrate fully. And so will you! Both of you have invested a lot of time and energy into this project. Go at your own pace and don't rush! Keep asking questions and spinning different ideas until the best decision has been found...right up to the bitter end.

And the champagne lunch that follows.