I'd say its fairly simple: my behaviour is what I do, but not what I am. So look at a business process, for example: clearly a piece of behaviour, i.e., what the business "does". The roles or actors (like me) are the structural things that perform this behaviour. So I can describe the behaviour in terms of processes, subprocesses, services delivered, etc., and those who perform that behaviour: the organisational units, individual actors, etc.  
 
BTW, Graham likes to add that in his view a use case definition is a behavioural model (a process), but a use case diagram is a structural model (relating actors to processes, with no sequential flow.) Just to show that his and my notions of behaviour and structure are also subtly different.