Protect Required Field Record For Free

On the page layout Click on Setup. Go to Quick Find and enter Object. Choose the object. Under the Page Layouts section, Click on Edit on the page layout that you use on the object. Click the wrench icon beside the custom field that you will make required. Select the Required checkbox. Click on Ok, then click on Save.

Tip You can't delete standard fields, but you can remove them from your page layouts. Navigate to the fields page for your object. On the field's information page, you can?depending on your edition?set field-level security, view accessibility settings, and configure validation rules.

Standard required fields are required. You can't make them optional. I have heard of people making Visualforce pages to set those fields to some value "behind the scenes", but they're still populated with some value. That's a lot of code to write just to add the convenience of a single field not being required.

Just need to go to Schema Builder and select the object. The list of fields can be scrolled through. The Required fields have a red mark on them.

It's possible to conditionally make a field required by using a validation rule.

Click on ?Insert Field? and scroll until you find the ?Current User? selection. From there, click on the ?Bypass VR? field. The field will now show in your validation rule. The validation should trigger when the field is FALSE.

Using Record Type & Page Layouts: Create two different record types say ?Open? & ?Locked? and change the record type of the record using workflow field update or trigger. And then create a separate page layout for the ?Locked? record type with all fields marked as 'Read-Only'.