'too solid' - 'don't move fast enough to catch the blur in the brain' before it's gone - yes that's sketching in the landscape with our changing English light. Fleeting flashes of sunlight or passing clouds on the sea or a hillside, pure drama . This is why so often I like to sketch in oils, it's possible to add the light back over darks, make major changes on the run. Trying to catch the blur in the brain.
Regular sketching plein air helps you build a visual vocabulary - a shorthand that helps you catch these things quickly - but never quickly enough!
Using a wide variety of marks makes the difference between the equivalent of cheapo Romantic Fiction and Literature. A varied language of marks in your head to draw on at need means it's easier to catch what interests you.
I encourage students to 'play' in sketchbooks trying out all sorts of ways of using their paints, combining them, mixing unusual combinations of media to see what happens, using tools other than paintbrushes like twigs, credit cards, the edges of card, spray bottles, combs, splattering, flicking paint, scratching through, pouring, adding collage, varying the speed at which marks are made - a mark made faster is freer than a slow and careful mark, experiment with this etc etc etc ...... once learned they can use these as appropriate (but never simply as gimmicks please! ).
And the silence so important - the quiet parts of a painting that allow the busier parts to shine. It's so easy to lose the quiet areas with overworking.
Painting and Drawing is a language like French or English - a vocabulary of marks, colour, tone, line, shape, balance, intervals , edges, mass, composition, surface ..... anything else? I'm sure there are things I haven't listed :>) The bigger your vocabulary to use at need the easier to express what you want :>)