Aiding an offence requires a positive act of assistance in its commission. This is not new law, but it is usefully illustrated in Robinson v R (Bermuda)  UKPC 3 (9 February 2011).
One of the ways in which the appellant had been alleged to have been guilty of the murders of twins was that he
"intentionally conveyed to … Burgess [the principal] by his presence and behaviour that he was assenting to and concurring in the commission of the offence"
Sir Anthony Hughes, for the Board, recognised the danger in this form of allegation :
"The Board is disposed to agree that to frame an allegation of aiding in [this way] does carry danger and is best avoided unless carefully qualified and explained. It courts the risk that insufficient attention is paid to the undoubted requirement that aiding imports a positive act of assistance. Of course that positive act of assistance may sometimes be constituted by D2 being present, and communicating to D1 not merely that he concurs in what D1 is doing, but that he is ready and willing to help in any way required. ... If D2's presence can properly be held to amount to communicating to D1 (whether expressly or by implication) that he is there to help in any way he can if the opportunity or need arises, that is perfectly capable of amounting to aiding ... . It is, however, important to make clear to juries that mere approval of (ie "assent" to, or "concurrence" in) the offence by a bystander who gives no assistance, does not without more amount to aiding. It is potentially misleading to formulate aiding [in the way mentioned above] without that qualification and without explaining that the communication of willingness to give active assistance is a minimum requirement."
The prosecution may prefer to allege aiding rather than the more complicated form of extended secondary liability. An opportunity to do this can occur where although a common plan may have been departed from, the aider continues to assist :
"... an aider (D2) is guilty in respect of acts which he assists the principal offender (D1) to commit, knowing what D1 is about, so that if D1 steps right outside what was contemplated by D2, the latter will not be guilty. That, however, assumes that D2's assistance ceases upon the fundamental departure by D1. It is clearly otherwise if D2 continues to render assistance after a change of direction by D1."
So, just a reminder of some fundamentals here.