The Collaborative International Dictionary of English v.0.48:

Interpose \In`ter*pose"\, v. t. [imp. & p. p. Interposed; p.
   pr. & vb. n. Interposing.] [F. interposer. See Inter-,
   and Pose, v. t.]
   [1913 Webster]
   1. To place between; as, to interpose a screen between the
      eye and the light.
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            Mountains interposed
            Make enemies of nations.              --Cowper.
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   2. To thrust; to intrude; to put between, either for aid or
      for troubling.
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            What watchful cares do interpose themselves
            Betwixt your eyes and night?          --Shak.
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            The common Father of mankind seasonably interposed
            his hand, and rescues miserable man.  --Woodward.
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   3. To introduce or inject between the parts of a conversation
      or argument. --Milton.
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The Collaborative International Dictionary of English v.0.48:

Interpose \In`ter*pose"\, v. i.
   1. To be or come between.
      [1913 Webster]

            Long hid by interposing hill or wood. --Cowper.
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   2. To step in between parties at variance; to mediate; as,
      the prince interposed and made peace. --Pope.
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   3. To utter a sentiment by way of interruption. --Boyle.

   Syn: To intervene; intercede; mediate; interfere;
        intermeddle.

   Usage: To Interpose, Intermeddle, Interfere. A man may
          often interpose with propriety in the concerns of
          others; he can never intermeddle without being
          impertinent or officious; nor can be interfere without
          being liable to the same charge, unless he has rights
          which are interfered with. "In our practical use,
          interference is something offensive. It is the pushing
          in of himself between two parties on the part of a
          third who was not asked, and is not thanked for his
          pains, and who, as the feeling of the word implies,
          had no business there; while interposition is employed
          to express the friendly, peacemaking mediation of one
          whom the act well became, and who, even if he was not
          specially invited thereunto, is still thanked for what
          he has done." --Trench.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Interpose \In"ter*pose\, n.
   Interposition. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

interpose
    v 1: be or come between; "An interposing thicket blocked their
         way"
    2: introduce; "God interposed death"
    3: to insert between other elements; "She interjected clever
       remarks" [syn: interject, come in, interpose, put in,
       throw in, inject]
    4: get involved, so as to alter or hinder an action, or through
       force or threat of force; "Why did the U.S. not intervene
       earlier in WW II?" [syn: intervene, step in, interfere,
       interpose]

Moby Thesaurus II by Grady Ward, 1.0:

96 Moby Thesaurus words for "interpose":
   act between, arbitrate, bargain, barge in, break in, break in upon,
   burst in, butt in, cast, charge in, come between, crash, crash in,
   crash the gates, creep in, crowd in, cut in, drag in, edge in,
   elbow in, encroach, entrench, fill in, foist in, fudge in,
   go between, horn in, impinge, implant in, impose, impose on,
   impose upon, infiltrate, infringe, inject in, insert, insert in,
   insinuate, insinuate in, intercalate, intercede, interfere,
   interjaculate, interject, interlope, intermeddle, intermediate,
   interpolate, intervene, introduce in, intrude, invade, irrupt,
   judge, lug in, make terms, meddle, mediate, meet halfway, moderate,
   negotiate, obtrude, press in, push, push in, put between, put on,
   put upon, referee, represent, run in, rush in, sandwich, shove,
   slink in, slip in, smash in, smuggle in, sneak in, squeeze in,
   steal in, step in, storm in, throng in, throw, throw in, thrust,
   thrust in, toss, treat with, trench, trespass, umpire, wedge in,
   work in, worm in