【Activate】 , 【actuate】 and 【motivate】 are sometimes confused when used with reference to persons and the motives which govern their actions. They are not interchangeable, however, because they carry divergent denotations.

Some external influence or agent, rather than a motive or desire, activates a person or thing when it supplies an effective stimulus to activity; the motive or, at least, a latent desire for such activity being commonly presupposed.

• Kapteyn′s work . . . was not final, but it . . . attracted and activated others
  —G. W. Gray
• A motive, a principle, or a desire actuates a person (not an action or undertaking) when it governs or determines his actions.
• the desire for conquest actuated the explorers of the sixteenth century
• individuals actuated by economic self-interest
  —Bush

A dramatist or novelist motivates the actions of his characters or the incidents of his plot when he supplies the motives for each.

• the novelist failed to 【motivate】 adequately his hero′s surrender to temptation

Also an objective, a desire, a passion motivates or gives the underlying motive of an action or undertaking.

• ambition motivated Macbeth′s murder of Duncan