Get output data without executing previous node

I see that you can get the input data of any previous node in the workflow in Expression Editor. However, is it possible to use an expression with the output data of a previous node without having previously executed it? I know that any previous node must be executed before a node down the line can see its output data in Expression Editor, but I wonder if I can directly just write an expression to use output data? So that I can just press workflow execute once and it’ll work.

Yes, you can write the expressions totally by yourself. The expression-editor is only there to make writing expressions easier and faster. But there is no difference at all between selecting it via the editor or writing it in by yourself.
You just have to know the syntax, the name of the node and the key.