Indeed, I've read some descriptions of such works where the menace seemed to ooze out of the description, and yet the character went on with the tradition Gandalf-like mode.
Which is a pity because the man who's come to sweep you away on the quest being of ill intent toward you, or even just indifference to your fate, would offer enormous plot possibilities. You've already got faction potential (assuming you allow the menace he speaks of to be genuine), and conflict, with the hero having to stand on his own two feet whenever he conflicts with the mentor -- and show off his heroism -- and the hero having to wrest out of him or someone else the truths he may be hiding to get his own way.
Of course, that's a problem too. One charm of the mentor is the ease of info-dumping. If you don't want the hero to spend his time wresting out the truth before he has his adventures, you would need another plot device to replace him. Humm -- the reason for the hero being chosen was his ability to read the magic book, blank to everyone else? That would really make the menacing mentor hate him. . .