It's a few years since I first made the acquaintance of Ms Samantha Mann, in her award-winning solo show Stories About Life, Death & A Rabbit. Since then, it seems, quite a lot has changed: she's cut down her work as a librarian, signed up for some further education, and diversified into a new career as an agony aunt. She grew up, she explains, in a family where it's "considered impolite to talk about things which aren't going very well"… so with the aid of a correspondence course and a YouTube channel, she's set out to redress that balance and encourage us to share the problems in our lives.

Ms Mann (she emphasises the "Ms") isn't real, of course. She's created by actor Charles Adrian, aided by a slightly unruly wig and an extremely practical jacket, and he proves eminently believable as a middle-aged woman. Adrian plays Ms Mann as a fussy, nervous character, flustered on stage and clearly awkward off it – but she's lovable too, and admirably willing to just give things a go. One hilarious scene, where she assembles a display stand with perfectly-observed incompetence, defines her character through sheer persistence in the face of physical adversity.

As we file into the theatre, Ms Mann invites us each to write a "dilemma" on a square of green card – which, after shuffling to anonymise their authors, she reads out and comments on towards the end of the show. Adrian treats these contributions with a the requisite degree of respect, but Ms Mann simply isn't cut out to be an agony aunt; time and again, she appears on course to say something balanced and supportive, only to spoil it with a naïve afterthought which reveals what she really thought all along. Her clumsiness and unconscious candour are both startling and endearing, and if this one basic joke is constantly repeated, that's only because it gets funnier and funnier each time.

Yet the irony is that, in blurting out truths no-one else dares mention, Ms Mann often gives the best possible advice. One laugh-out-loud sequence sees her run through frequently-asked questions, sorting them into categories as all good librarians should: and her chosen headings, things like "Just talk to them!", deftly illustrate how often we turn straightforward situations into intractable dilemmas. If anyone else dispensed this advice, it would sound rudely simplistic – but Ms Mann seems genuinely puzzled that anyone could overlook something so obvious, and thereby delivers some genuinely important lessons for life.

Behind the Agony works perfectly well as a stand-alone show, but if you saw Stories About Life, Death & A Rabbit there's an extra dimension to the monologue. The multi-threaded back-story of the last show is necessarily stripped back here, but there are touching moments – most strikingly, a lingering glance at a photo of her brother. There are in-jokes to test your memory of what you learned last time, and a lot of pleasure to be had in seeing how Ms Mann has moved on from the crossroads she seemed to have reached back then. She's made a couple of unexpected leaps, says a couple of things that don't quite fit the character I'd remembered... but real people often surprise me, so there's no reason why a fictitious persona shouldn't go through some changes too.

Charles Adrian's great triumph is that I believe in Samantha Mann. I believe in his performance – to the extent that I struggle to internalise that this fully-fleshed, complex character isn't actually real. But I believe in her in another sense as well: root for her to achieve her ambition, to break out of the shell she's put herself in and find a new joy in her life. She isn't a natural agony aunt, but she'll give it her very best shot. We can all learn from that example.