The sense of what is appropriate and inappropriate practice is only a sense and does not come in the form of rules. Given particular stimuli which we are trained to recognize, we do not respond by following a rule which dictates exactly what it is we can do but only a guideline which sets the parameters of what is appropriate and what is inappropriate practice.
People who have a good feel for what is appropriate and inappropriate practice are capable of coming up with inventive responses to stimuli because they have a good sense of how far practice can be stretched without going too far. People who do not have a good feel for what is appropriate and inappropriate tend to give the standard responses as if they were following a rule. In colloquial terms, they are stiff and are often advised to relax.
The other possibility is that people who are considered by some to be stiff simply have a different sense of what is appropriate and what is inappropriate behavior. They simply appear to be complying with a rule but in reality they are doing something which comes naturally to them. In other words, they are only stiff from the perspective of those who call them stiff but from their own point of view, they are not stiff at all.