For each of your main characters, create a one-page profile that describes their physical attributes as well as their occupations, education level, family status, living conditions, wants, needs, hopes, fears, and personal values. Identify their favorite foods, music, movies, books, what makes them cry, what makes them laugh, and who---or what---is the strongest influence in their lives.
The hero and the villain apply for the same job and are the two final candidates being considered. After identifying what the job is and why they each want it, write two separate scenes of dialogue in which they respectively give their most persuasive pitches to the employer on why they should be hired over the competition.
The main character is embarrassed or ashamed about an incident that occurred in the past and decides that the time has come to sit down and write a letter of explanation and apology. Identify the recipient of this correspondence along with the sender's expectations of understanding and/or forgiveness. Special points to consider are why the sender took so long and what has transpired in this character's life in the interim.
Through a glitch in the time/space continuum, the lead character finds himself bouncing from the Wild West to the Dark Ages to Ancient Egypt to 19th century England. Knowing what to do of his personality, intelligence and skill sets, identify what jobs he'd be best suited for in each of these diverse environments as he struggles to find his way back to the present. Consider how he'd describe his experiences in a journal.
You're a newspaper reporter assigned to interview the hero or villain of your book for a notable accomplishment for a feature story. Compose a list of 20 in-depth questions your readership want to know, then compose the answers your character gives in a "voice" that is different and distinct from your own.
Choose any character in your novel to imbue with super powers. In addition to deciding what the super power is---and what kind of costume is involved---examine how this impacts the recipient's relationships with others, outlook on life, self-esteem and willingness to take risks. Record these impressions as a diary entry written in the first-person.