I'll give you the long, nerdy take because word choices are my little hobby. When a character 'lights up' in dialogue, the best synonym depends on whether you mean literal illumination or that metaphorical burst of feeling. For a literal sense — a lamp, a screen, a room — 'illuminate' works but sounds a touch formal in casual speech. More natural swaps are 'brighten', '
glow', 'flood (with light)', or 'bathe' (as in 'The hall was bathed in sunlight'). For a face or eyes lighting up with joy, I reach for verbs that carry emotion: 'beam', 'brighten', 'glow', '
smile spread', or even 'her eyes shone'.
If I'm writing dialogue and want it to sound conversational, I try short, punchy verbs. Instead of 'He lit up when he saw the dog,' I'll often write 'He beamed when he saw the dog' or 'His face brightened when the dog bounded over.' For more lyrical or atmospheric moments, 'illuminate' or 'bathe' can be lovely: 'The streetlights illuminated her path.' And for intellectual or emotional revelation — replacing 'light up' meaning 'be enlightened' — 'enlighten', 'clarify', or 'reveal' are better fits.
I like to match rhythm and character voice: a teenager might 'light up' or 'beam', an older narrator might 'brighten' or 'illuminate', and someone poetic might 'glow' or say the room was 'flooded with light.' Try a couple of options aloud and pick what feels true to the speaker; that always helps me pick the right shade of meaning.