Mo. Rev. Stat. § 491.030: Adverse party may be compelled to testify in civil cases.

Any party to any civil action or proceeding may compel any adverse party, or any person for whose immediate and adverse benefit such action or proceeding is instituted, prosecuted or defended, to testify as a witness in his behalf, in the same manner and subject to the same rules as other witnesses; provided, that the party so called to testify may be examined by the opposite party, under the rules applicable to the cross-examination of witnesses.