The nature of a ‘common design’ was explained by Mustill LJ in Unilever v Gillette  RPC 583, at p 609:
‘I use the words common design because they are readily to hand but there are other expressions in the cases, such as ‘concerted action’ or ‘agreed on common action’ which will serve just as well. The words are not to be construed as if they formed part of a statute. They all convey the same idea. This idea does not, as it seems to me, call for any finding that the secondary party has explicitly mapped out a plan with the primary offender. Their tacit agreement will be sufficient. Nor, as it seems to me, is there any need for a common design to infringe. It is enough if the parties combine to secure the doing of acts which in the event prove to be infringements.’
The individual needs to join or share in the commission of the tort which generally means some act which at least facilitates its commission.
In addtion the law relating to this species of joint tortfeasance was explained by the Supreme Court in Fish & Fish Ltd v Sea Shepherd UK Ltd  UKSC 10;  AC 1229, and in Vertical Leisure Ltd v Poleplus Ltd  EWHC 841 (IPEC) where Hacon stated :
“I interpret this to mean that in order to fix an alleged tortfeasor with liability, it must be shown both that he actively co-operated to bring about the act of the primary tortfeasors and also that he intended that his co-operation would help to bring about that act (the act found to be tortious). Liability will always be subject to the threshold requirement that the alleged joint tortfeasor’s contribution to the act was more than de minimis.”