Often, when actors give less-than-adequate performance in a scene, they place the blame on their scene partner: "They (the other actor) wasn't giving me enough to react to." The complaining actor's implicit assumption is that their less-than-adequate performance, especially in manifesting emotion, is tied to the form and quantity of their scene partner's stimulation.
To such complaint, I say, yes...and no.
A truly fine actor can be stimulated into their own marvelous performance by very wide quality and quantity range of partner initiation. The important consideration in an actor giving a great performance is the sensitivity and flexibility of his own emotional condition.
Let me cite an example from everyday life (which, once again, what acting mirrors): I want to have a good time. If I am truly open to having a good time, I can date anybody...and I can have a good time. I can have a good time with an ignorant person ("It'll be fun teaching them"), a smart person ("It'll be fun learning from them"), a beautiful person ("It'll be fun trying to make love to them"), a homely person ("It'll be fun to convince them they are beautiful"), etc.
To have a good time is ultimately not dependent on them, the others, but on myself' my emotional success, my open receptivity to them (in this case, my open receptivity to having a good time irrespective to the particulars of what they give me). Granted, each good time with each different person will be a little different, but the general emotional fact of having a 'good time' will be irreducible and constant irrespective of the person so engaged.
The same possibility holds with being made sad, or silly, or angry. For example, if I am in 'an angry mood', I have found anything and everything someone makes me angry: a smile ("How dare you smile at me when I am in this mood?"), a contradiction ("How dare you disagree with me?!"), an agreement ("How dare you agree with me; I know you're just placating me!"), an expression of love ("haw dare you lie to me!!"). The same constancy of stimulation holds for silliness, or sadness. If I get deeply into an emotional 'mood', whatever the mood, all stimuli I receive from the other actor, whatever the quality and quantity of their stimuli, will cause in me a deep and specific emotional response.
Finally, I don't even need an actor's particular look or sound of voice to stimulate me. (Cold reading, for example, with a casting director has often been described as indulging in a form of acting necrophilia: getting off on the dead.) So I respond to that reality of that minimum stimulation by accepting: words themselves can move me deeply. I have cried, or laughed, or been puzzled, or been made angry by a letter: nothing but words, without a living voice or face.
The best answer to an actor's complaint vis-a-vis the the inadequacy of their performance partners, is: "The fault, dear Brutus, lies not with the stars, but (all too often) within we ourselves."