Coercion can take many forms. Threats of physical violence or, indeed, actual violence could be undue influence. Psychological or emotional pressure applied to a will maker might be undue influence.
This is particularly true if the will maker was suffering from a serious illness and had no power to resist the pressure.
Undue influence must be considered on a case-by-case basis. What might be undue influence for an ill, enfeebled will maker might not be undue influence for a healthy, strong-willed person.
In short, undue influence has occurred if, at the time of making his or her will, the will maker might say, “this is not my wish, but I must do it.”
Evidence of undue influence is rare, and difficult to obtain. The influencer is unlikely to, for example, put in writing or say in public that the will maker must make his or her will a certain way. This difficulty with evidence means that the court is particularly cautious when considering undue influence claims.