for example
我想吓你。 → I want to scare you.
你吓着我了。 → you scared me.
“吓”is just a verb,“吓着” emphasize the behavior has been completed.
by the way,“着”has two pronunciations,one is【zháo】,the other one is 【zhe】
着【zhe】means “doing”,我吃着【zhe】寿司了→ i'm eating sushi now.
着【zháo】means “done”,我吃着【zháo】寿司了→ i have eaten sushi already.