跃然 almost always appears in the set patterns 跃然纸上 (leaping off the page - for vivid writing) and 跃然...上/间 (vividly showing on...). It's a literary word - you'll see it in book reviews, art criticism, and essays, but rarely in spoken Chinese.
memory
跃 = to leap; 然 = a classical adverb suffix meaning 'in such a manner'. So 跃然 literally is 'in a leaping way' - picture a description so lifelike the figure jumps out of the page at you.
