The Loyalty Oath during the Civil War

[Anita]

Who had to sign the loyalty oath and how effective was it? Were background checks done? What were the consequences if one broke the oath?

I know government employees had to take the oath and some businesses. Then there were those crossing North/South boundaries. There are many notes signed by Lincoln "Let him/ her take the oath".

Mary Lincoln's half-sister Emilie refused to sign the oath in 1864. Why didn't actors such as Booth have to take the oath or those traveling to Canada?

Lastly, was there a legal issue?

I'd appreciate clarification as I find it quite confusing. Thanks.