Let's say my company purchases some tangible/fixed/long-term asset. Am I required to take a prorated depreciation expense each year for the life of the asset? Do I have the option to report the expense as a lump-sum/one-time expense on my income statement? Is it up to me to decide what I want my earnings to look like that year (i.e. lower if I take the one-time expense and higher using a depreciation expense)? Or is all of this somehow dependent on whether I am using GAAP?
For public US companies listed on an exchange, they must follow SEC guidelines. Thus, their financial statements will be prepared under US GAAP.
To answer your question, yes, if the cost is over their capitalization policy amount, it must be capitalized and depreciated.
Under GAAP there are several options for depreciation, such as straight line and unit-of-production. The choice is always disclosed in the footnotes of the 10-K. Ideally the company would choose a method that resembles the actual depreciation of the asset most closely so the statements are more reliable.