The "old Hollywood way" of covering an actor's path with even light is to back the key light up and use single and double open end nets to "feather" the light output as the actor comes closer. Sometimes a grip will also "float" a net into the light to effectively "dim" the output as the actor approaches.
maybe your actor could walk through some pool of lights. or maybe you could throw a couple of lights through some doorways and your actor passes by them. but if you need to constantly see your actors face then you could do what John says. a great little light is the mini lite panel that is battery operated.