Most people who have watched Glenda Jackson's 23 years as an MP would agree that courage isn't something she's short on. So when she complained last year about the dearth of opportunities for older actresses, blaming in part classical drama's deficiency of good female parts, we might have guessed what was to come. For her return to the stage after two decades, the 80 year-old Oscar winner is taking on King Lear.

There is much to praise in her performance, not least her astounding stamina in tackling the production's three-and-a-half-hour run time. She steams out of the blocks, hissing her way through her rejection of Cordelia, howling at her own mistreatment – all with a captivating commitment. But there are problems, too. With such a diminutive frame – admittedly a poignant attribute for the ageing Lear – her voice is her chief currency with which to show the King's initial authority but, at times, it lacks power and flexibility. Later, however, her stature and quieter vocal approach come into their own, combining to give a beautiful gentleness to Lear's descent into madness – it's like watching a child carefully building a protective den of blankets against their bedroom monsters.

A female Lear isn't the only daring choice in Deborah Warner's production; modernity and metatheatricality shine from the stage throughout. The set is a blazing white box of light, which, as the audience enters, teems with technical crew adjusting huge video screens labelled with their dimensions and functions. Actors chat and leaf through scripts. While exposing the play's theatricality is an interesting choice – perhaps a reference to our (and Lear's) tendency to believe what we want to – it could be pushed further. With the stagehands only returning in the interval, it seems dangerously close to a gimmick. Warner does, however, make some ingeniously simple choices, from a black tarpaulin curtain covered in projections of rain that lashes as the storm rages, to a backlit white sheet that intensifies the horror of the eye-gauging scene by creating a creepy accompanying shadow-puppet show.

Her inventiveness when it comes to her actors is also undeniable. There are many perfectly-pitched moments but a crowd favourite was Edmond's speech about the supposed inferiority of bastards, which he delivers while performing feats of physical prowess.

The actor in question, Simon Manyonda, is one of the strongest in the excellent star-filled cast, which also includes a dangerously oversexed Jane Horrocks as Regan and a briskly callous Celia Imrie as Goneril. Rhys Ifans brings welcome levity as Lear's Fool, delivering some winning contemporary gags, the best being a nauseating reference to Wahaca's recent Norovirus outbreak.

For all the brilliance on stage, however, the production's length proves a problem. By the closing scenes, audience members were fidgeting, meaning the final flurry of deaths seemed to come to them as a welcome relief rather than a tragic climax. Perhaps further cuts will be introduced. If not, take a snack for the interval. There is a much to marvel at if you can maintain your focus.

King Lear runs at the Old Vic until 3 December