Julie Walters as Mary Whitehouse. Photograph: BBC

Tomorrow, Julie Walters gives a cracking performance in Filth: the Mary Whitehouse Story on BBC2 - looking alarmingly like Prunella Scales. It occurred to me that Walters is one of those actresses whose presence in a drama seems to be a mark of quality, indicative that a treat is in store. In this respect, Walters is, I think, like Judi Dench.

Not everything Walters does is brilliant - her turns in Ahead of the Class (inspirational headmistress with an accent of indeterminate Celtic-ness) and in The Return (as a ex-alcoholic who may or may not have murdered her husband) were hardly outstanding - but such bumps in the road almost immediately forgiven and forgotten while Walters' incredible performances - that of grieving mother Angela in Abi Morgan's brilliant Murder - are emblazoned on the memory.

In Dame Judi's case, Cranford must now be collapsing under heaped accolades but, though I had something of a soft spot for As Time Goes By, it was hardly a high spot in her oeuvre. Ditto and more so with The Chronicles of Riddick.

Whose presence in a drama do you take as an indicator of quality (even if it isn't really a guarantee)? Julie Walters even makes me want to shop at Asda - and believe me, that takes some doing.