To determine character motivation, analyze each character's goals and conflicts. Ask: What is it that each character wants? The ask: What is it that prevents each character from attaining that goal? From this will follow the scheme or stratagem or motivation within the acting.
For example, Lady Chiltern's goal is to act with perfect goodness and to revere her husband, as he too acts with perfect goodness. Things go along swimmingly for Lady Chiltern until something occurs that threatens to disillusion her regarding her husband's perfect goodness.
From this information, the acting motivation, or scheme, can be established. Lady Chiltern's motivation (reason) for each of her actions, each of her lines can be determined and developed based on her goal and the conflict keeping her from that goal.
As another example, Lord Goring's goal is to honor his friendship with Sir Robert Chiltern. This means that he must also preserve his friend from ruin--should ruin threaten. And ruin does threaten in the form of a blackmail attempt by Lady Cheveley.
Lady Cheveley's blackmail attempt produces the conflict that threatens to keep Lord Goring from his goal and which he must resolve. So Goring's motivation is to do what will attain his goal while resolving the conflict.