Indiana Code Title 34. Civil Law and Procedure § 34-45-2-10

Sec. 10. (a) In all cases in which:

the assignor or grantor of a party making the assignment or grant voluntarily shall be considered a party adverse to the executor or administrator, heir, or devisee.

(b) However, in all cases referred to in sections 4 through 9 of this chapter, any party to the suit has the right to call and examine any adverse party as a witness.

(c) The court may require any party to a suit or other person to testify.  Any abuse of the court's discretion under this subsection is reviewable on appeal.