Jessica Chastain's Blonde Bob: The Cut, Colour and Products to Show Your Stylist

It is a chin-grazing bob with a blunt, heavy perimeter and almost no visible layering. The colour is a cool, creamy platinum rather than a stark white, with a slightly deeper root that stops it looking flat. The finish is glossy and smooth, tucked behind one ear. It looks expensive because it is so precise, and that precision is what to ask for.

It suits oval and heart-shaped faces most easily. A rounder face looks better with the length dropped half an inch below the chin and a side parting. Fine hair loves a blunt bob, since the solid line makes it look thicker. Very thick hair needs internal thinning so the shape doesn't puff out into a triangle.

Ask for a one-length bob cut to the jawline, with a blunt perimeter and weight kept at the ends. Request a deep side parting and a little invisible texturising inside if your hair is thick. Say clearly that you don't want visible layers or a graduated back. Bring several pictures from different angles, because stylists read the back and sides as much as the front.

This is the big commitment. Going from red or brunette to platinum usually takes more than one appointment, a bond-building treatment such as Olaplex or K18 during bleaching, and a toner to remove the yellow. Expect to spend a few hundred pounds at a good salon and to go back for the roots every five to six weeks. Natural redheads tend to lift to a stubborn orange, so be patient.

Always ask for a skin allergy test 48 hours before, and a strand test to check your hair can take it. If it has been coloured dark several times, a good colourist may suggest a buttery blonde first. That is good advice, not a lack of ambition.

A purple shampoo from Boots once a week keeps brassiness away. Not every wash, or the blonde turns lilac. Olaplex No.3 and the K18 leave-in mask repair bleached hair between appointments. A lightweight hair oil from Space NK on the ends gives the glassy finish. And a bob this blunt needs smoothing, so a pair of GHDs and a heat protector are worth having.

Chastain didn't dye her hair, so there's no shame in doing the same. A good-quality bob wig is a brilliant Halloween or party option, and plenty of people now wear them for fun. Alternatively, keep your own colour and just take the cut. A blunt jaw-length bob looks just as sharp in copper, chocolate or honey.

A bob needs a trim every six to eight weeks to keep the line sharp, and platinum needs regular toning. Factor both into the cost. Growing it out is slow, so be sure. If going lighter feels like too much, see why copper is the colour of the season instead.