In two speeches, Fainall is characterized by himself and by Mirabell. Fainall describes himself, in our terms, as an opportunist, a man who can veer with the winds of circumstance. Mirabell describes him as a man on the fringes of respectability, a man who is almost acceptable. To these two complementary descriptions we must add another quality noted before — Fainall's intense suspicion. He distrusts his mistress as naturally as he breathes; he distrusts everything Mirabell says. It is not that he assumes Mirabell is lying, necessarily; rather he looks for snide implications in the words and finds them. In justice to Fainall, it should be noted that the snide implications are there.
The one disreputable act we can attribute to him before the play starts is his marriage. The fact that he married for money can hardly be held against him in his society, but to marry for money to finance a love affair is more difficult to accept. Yet it is hard to see that his part in marrying the rich widow is worse than Mirabell's in arranging for the marriage of his mistress to his friend so as to protect her from scandal should she become pregnant through his, the lover's, attentions.
In each of the items mentioned above, Fainall is a somewhat tarnished version of Mirabell. Mirabell's deftness in handling his world becomes Fainall's "bustling" opportunism. Mirabell's caution in trusting people becomes Fainall's almost pathological suspicion of every word anyone says.
It is in their loves that we can see, glaringly, Fainall's attitude to life as a smirched version of Mirabell's. Possibly against their wills, both are in love. Mirabell moves to a marriage based on mutual respect. Fainall will try to shut his eyes to what he sees and pretend to believe against clear evidence in a love affair hemmed in on all sides by indignity and deceit.
Come, I ask your pardon — no tears — I was to blame, I could not love you and be easy in my doubts. Pray, forbear — I believe you: I'm convinced I've done you wrong, and any way, every way will make amends. I'll hate my wife, yet more, damn her! I'll part with her, rob her of all she's worth, and we'll retire somewhere — anywhere — to another world.
When Fainall's suspicions about his wife are confirmed, he moves from a kind of generalized unpleasantness to quite specific action. Once his plans are made, he proceeds ruthlessly.