Boost docs similar to top result of query

Hi guys,

I'd like to know how can I boost docs based on their similarity to the top result of the query according to a field, despite not matching with the query itself.

Imagine this situation: I have an index of movies with fields "name" , "actors".
Someone searches for "titanic". My top result is titanic movie, but I'd like to boost other movies that share an actor, for example "Memento" because it also has "DiCaprio" in the actor field.

How can I do this without having to reindex extra fields?
I'm not even finding the right mechanism for this, but im fairly new to elastic.

Thank you so much!